Magnetic resonance imaging (MRI) is emerging as a powerful tool for the imaging of breast abnormalities. In general, MRI provides a better characterization of the breast lesions than conventional imaging modalities due to rich soft-tissue contrast, thin-section and multiplanar capabilities.
Traditionally, lesion morphology is analyzed and classified to discriminate benign lesions from possible cancer tumors. For example, American College of Radiology (ACR) has over the years developed a set of characteristics and lexicon for use with Breast Imaging Reporting and Data systems (BI-RADS®). BI-RADS MRI lexicon suggests that the following morphological features are likely associated with benign lesions:
Shaperounded, oval or lobularMarginsmoothMass enhancementhomogeneous, no contrast enhancement,non-enhancing internal septation
On the other hand, the BI-RADS MRI lexicon suggests that the following features are likely describing the possibility of malignancy:
ShapeirregularMarginspiculatedMass enhancementheterogeneous, rim enhancement, ductalenhancement
Recently, considerable attention has been focused on contrast-enhanced MRI of breast lesions. Before or during the exam, a contrast enhancement agent is injected into a vein in a patient's arm. Typically, a gadolinium based contrast agent (e.g., Gd-DTPA) is used. The use of contrast agents tends to provide greater contrast between normal and abnormal tissues. The contrast enhancement stems from the fact that the growth and metastatic potential of tumors can be directly linked to the extent of surrounding angiogenesis. For a tumor to grow larger than a few millimeters in diameter, it requires the formation of blood vessels that will supply oxygen and nutrients necessary for survival. These new vessels proliferate in a disorganized manner and are poor quality, thus making them leaky and causing blood to pool around the tumor. The analysis of the signal from diffusible contrast agents aids in the detection and characterization of suspicious abnormalities in breasts.
Quantitative studies of the signal intensity over time (or “kinetics curve”), as time variation of level of enhancement and the kinetics (e.g., uptake and washout behaviors), suggest that a malignant lesion is likely an area that enhances rapidly, reaching their peak enhancement between one and three minutes post injection. Benign lesions enhance more slowly, with the peak enhancement occurring after several minutes.
The shape of a kinetics curve also can be a good indicator whether a lesion is malignant. Studies have found that kinetics curves describing a benign lesion tend to be straight or slightly curved (type I). For the curved type, the time-signal intensity continue to increase but the growth is generally slower and the curve is flattened in the late post-contrast period, because of saturation effects. On the other hand, kinetics curves that suggest or indicate malignancy show a plateau or a washout section. The plateau type (type II) shows an initial upstroke, after which enhancement is abruptly cut off, and the signal intensity plateaus in the intermediate and late post-contrast periods. The washout type (type III) shows an initial upstroke, after which enhancement is abruptly cut off, and the signal intensities decreases (washes out) in the intermediate post-contrast period (2-3 minutes after injection of contrast agent).
However, although the contrast-enhanced MRI method has achieved high levels of sensitivity (94%-100%), it provides only limited specificity levels (40%-95%). Here, sensitivity refers to true positive detection and specificity refers to false positive reduction. The low specificity levels are result of not only malignant lesions enhancement but also enhancement of the benign lesions, causing a number of unnecessary biopsies. Thus, the presence of enhancement alone cannot be used to differentiate benign from malignant lesions.
Benign lesions are regarded as results of aberrations of normal processes. For example, fibrocystic lesions are the most common benign disorder (40%-50%), fibroadenoma is the most frequent tumor in young and adolescent woman, and pappiloma is a low risk lesion. Other benign lesions include radial scar (sclerosis), which is a stellate lesion mimicking cancer, phyllodes tumor, and ductal hyperplasia.
Investigations of contrast MRI of breasts have demonstrated that not only did malignant lesion enhance, but also many benign lesions including fibroadenomas, fibrocystic changes and radial scars enhanced. Also, there may be malignant lesions, such as certain cases of infiltrating ductal carcinoma (IDC), infiltrating lobular carcinoma (ILC) or ductal carcinoma in situ (DCIS) that will not enhance rapidly but in which lesion morphology suggests the presence of malignancy. The belief is that the presence of contrast enhancement alone cannot be used to differentiate benign from malignant lesion.
Recently, attention has also been turned to magnetic resonance spectroscopy (“MRS”) as a new technique for diagnosing cancer. MRS is a particular type of magnetic resonance detection technique. It provides chemical information by measuring concentrations or strengths of various marker chemicals, such as choline, in a suspected tumor. It is believed that the amount or concentration of marker chemicals provide information about the disease process in the area examined.
In general, signals obtained from MRS do not generate a scanned image. Instead, spectroscopic information of various chemicals is produced. More recently, it has been possible to obtain spectroscopic data from a well localized area. This allows the biochemical information obtained from MRS to be evaluated in relation to the localized area. However, correlating spectroscopic data with a scanned image is generally a difficult task in clinical environments.
The forgoing creates challenges for developing a system and method of analyzing medical images for discriminating between malignant and benign lesions, suitable for clinical needs. It is an object of the present invention to mitigate or obviate at least one of the above mentioned disadvantages.